Yeah, what Davey said. Heaters are notorious for malfunctioning, and I would not trust one without a controller. I set the heater a degree or two above the temp set on the controller, so even if the controller managed to get stuck on, the heater would still shut off. And, if the heater itself got stuck on(been known to happen), then the controller would shut it off.
This can also be used for a chiller. This model is a single stage controller, so it can control either a heater or a chiller, but not both. I bought a dual stage as my system is now in the garage, and needs the heater at night and the chiller during the day, and I prefer to have a single controller, as opposed to one for each unit.